1. Field of the Invention
The present invention relates to a medium conveyance apparatus, an image forming apparatus and a medium conveyance method, and more particularly to medium fastening technology for conveying a medium in a prescribed direction while maintaining the attitude of the medium.
2. Description of the Related Art
A general image forming apparatus known in the related art is an inkjet recording apparatus which forms a desired image on a recording medium by ejecting color inks from nozzles of an inkjet head while conveying a fastened recording medium in a prescribed conveyance direction.
In general, in order to form images of high definition, it is necessary for the inkjet head and the recording medium to be situated in closest possible proximity during image formation. However, if the recording medium makes contact with the inkjet head due to the inkjet head and the recording medium being situated in close proximity, then not only does the recording medium become soiled, but the inkjet head may also be damaged. Therefore, in order to prevent contact between the recording medium and the inkjet head, an extremely small throw distance of several millimeters or less is provided between the inkjet head and the recording medium.
On the other hand, when the recording medium passes directly below the inkjet head, if the recording medium or the structure arranged on a recording medium supporting surface make contact with a nozzle face of the inkjet head, then a liquid repelling film formed on the nozzle face is damaged, foreign matter adheres to the vicinity of the nozzles, and the ink ejection performance declines. It is then necessary to convey the recording medium in such a manner that the nozzle face and the recording medium, and the like, do not make contact.
Japanese Patent Application Publication No. 2009-279795 discloses a composition in which a recording medium is conveyed in a state where the leading end portion thereof is gripped by a gripper arranged on a conveyance drum. In this composition, by gripping the leading end portion of the recording medium by means of the gripper, the leading end portion of the recording medium is prevented from bending to become in contact with the nozzle face, and furthermore, by disposing the gripper to the inner side of the circumferential surface of the conveyance drum, the gripper is prevented from becoming in contact with the nozzle face.
However, when the leading end portion of the recording medium is held by a holding member, such as the gripper, and a portion other than the leading end portion of the recording medium is fastened on the circumferential surface of the conveyance drum by an air suction method, or the like, then the recording medium may be conveyed in a distorted state in the leading end portion of the recording medium, or at the boundary between the leading end portion and the other portions of the recording medium. Due to the occurrence of distortion of this kind in the recording medium, there is a high possibility of the recording medium making contact with the nozzle face.
If a gripper is disposed to the inner side of the circumferential surface of the conveyance drum, as in the composition disclosed in Japanese Patent Application Publication No. 2009-279795, then distortion is liable to occur when the portions other than the leading end portion are caused to adhere tightly to the circumferential surface of the conveyance drum by a pressing roller, or the like, after the leading end portion of the recording medium has been gripped by a gripper. In particular, marked distortion appears when using a recording medium of large thickness.
Japanese Patent Application Publication No. 2002-292956 discloses a conveyance drum type conveyance apparatus equipped with a holding device that holds a front edge of cut sheet of printing paper in a printing unit having a NIP (non-impact printing) head. This holding device is composed in such a manner that when printing paper is received from a paper supply drum, the front edge of the cut sheet of printing paper is held between a clamp jaw aligned in parallel with the axial direction of the conveyance drum and the front edge of a cut sheet supporting surface, and when the holding device approaches the NIP printing head, a pressing hook device opens and the clamp jaw is moved inside the conveyance drum, so as to prevent interference between the NIP printing head and the clamp jaw. With this composition, however, the holding mechanism of the leading end portion of the recording medium is opened before the recording medium enters directly below the NIP printing head (inkjet head), and therefore the recording medium becomes significantly separated due to the occurrence of floating up in the leading end portion of the recording medium, and there is a raised possibility of contact between the recording medium and the nozzle face.